1. Field of the Invention
This invention enables or provides for efficient, rapid, wireless communication of drilling information along a drillstring, while drilling is in progress, to allow optimal control of drilling direction and other drilling parameters. In particular, it provides a method for both injecting electrical currents into, and receiving electrical currents from the drilling mud in a borehole and from formations surrounding a drillstring with high efficiency and low propagation loss. In general, it relates to the field of conformal, surface mounted signal transmission and reception electrodes. The compact nature of the electrode apparatus and method allows for communication between any bottom hole assembly components where a wire or large transceiver mechanizations are not practical or possible.
2. Prior Art
Directional drilling of boreholes is a well known practice in the oil and gas industries and is used to place the borehole in a specific location in the earth. Present practice in directional drilling includes the use of a specially designed bottom hole assembly (BHA) in the drill string which includes a drill bit, stabilizers, bent subs, drill collars, rotary steerable and/or a turbine motor (mud motor) that is used to turn the drill bit. In addition to the BHA, a set of sensors and instrumentation, known as a measure while drilling system (MWD), is required to provide information to the driller that is necessary to guide and safely drill the borehole. Due to the mechanical complexity and the limited space in and around the BHA and mud motor, the MWD is typically placed at least 50 feet from the bit above the motor assembly. A communication link to the surface is typically established by the MWD system using one or more means such as a wireline connection, mud pulse telemetry or electromagnetic wireless transmission. Because of the 50 foot. lag between the bit location and the sensors monitoring the progress of the drilling, the driller at the surface may not be immediately aware that the bit is deviating from the desired direction or that an unsafe condition has occurred. For this reason, drilling equipment providers have worked to provide a means of locating some or all of the sensors and instrumentation in the limited physical space in or below the motor assembly and therefore closer to the drill bit while maintaining the surface telemetry system above the motor assembly.
One of the primary problems that must be overcome to locate sensors below the mud motor is the establishment of a communications link that can span the physical distance across the mud motor and be compatible with the construction of the mud motor and BHA. Prior art exists using three basic technological means, wired conduction through the mud motor, acoustic transmission and finally wireless electromagnetic communication.
An example of prior wired conduction art is U.S. Pat. No. 5,456,106 (Harvey, et al), which describes a modular sensor assembly located within the outer case of a downhole mud motor between the stator assembly of such a motor and the lower end of the outer case, where radial and thrust bearings are located. This sensor assembly is connected to a region above the stator by a wire mounted in the outer motor case.
U.S. Pat. No. 5,725,061 (Van Steenwyk, et al) is another example of a non-telemetry method of getting near-bit sensor data through a mud motor. This describes a way to run signal wires through the rotor of the motor, with slip-ring type electrical contacts at each end of the motor.
Wires allow transmission of both electrical power and signal data, but are mechanically difficult to implement and electrically maintain in the downhole environment and are not widely used due to these deficiencies.
An example of an acoustic based transmission system applied to a short hop application is described in U.S. Pat. No. 5,924,499 (Birchak et al). An array of acoustic transmitters is described that can pass signal through multiple paths to a receiver wired to the MWD system located above the motor assembly.
The complexity of this systems in terms of the mechanical packaging of the acoustic transmitters and receivers as well as the complex signal processing necessary to decode signals in the presence of the large acoustic noise inherent in drilling makes this method costly and prone to reliability problems.
Wireless electromagnetic communication on drilling assemblies has a long history of prior art starting with U.S. Pat. No. 2,354,887 (Silverman et al) which describes a toroid core with a primary winding wound on the core and the drill string located through the center opening of the toroid producing a one turn secondary. Current is induced in the drill string which travels to the surface where a potential difference is measured as the current returns through the earth.
U.S. Pat. No. 5,160,925 (Daily et al) uses a similar toroid method for both launching and receiving the signal in the drill string. Such toroids have the disadvantage of being thick cross-section structures (for both strength in the high-vibration drilling environment, and to avoid permeability saturation), and that they must be shielded from abrasion due to contact with the mud/borehole walls. These requirements mean that a deep groove, usually about one inch in depth, must be cut around the outside wall of the sub or other drillstring element hosting the toroid. This substantially weakens the element, already subject to high torque and bending forces, especially near the bit. Secondly, the toroid must be constructed as a split ring to fit over the host structure, wound with wire, and then reassembled in place to precision tolerances (to avoid high coupling losses). It must finally be encapsulated with an insulating polymer to hold it in place, and covered with a complex, slotted steel shield. All this makes use of the toroid method expensive as well as creating more potential points of failure due to the complex structure required for packaging.
A second type of wireless electromagnetic communication as described in U.S. Pat. No. 6,057,784 (Schaaf et al) comprises a solenoid coil wound about a center line of the drill string axis either on a separate drill string sub or as part of the bit box of the drill bit. A plurality of ferrite bars distributed about the inner circumference of the coil embedded in the body of the transmitter sub enhance the launching of the magnetic field into the drill assembly, surrounding borehole and earth. Surrounding the outer diameter of the coil is a slotted shield which provides protection from the borehole environment while allowing a propagation path for the magnetic field. Located above the mud motor, a second solenoid assembly similar or identical to the transmitter receives the signal in the reciprocal process used to launch the magnetic field As with the toroid method described in U.S. Pat. No. 5,160,925, the transmitter and receiver described in U.S. Pat. No. 6,057,784 are complex and therefore costly to maintain and manufacture.
All of the prior art methods describe complicated mechanical structures using a large number of parts and assemblies for construction of the transmitter and receiver. Due to the large cross section required to house them, the large coils and magnetic components described in the prior art reduce the strength of the bit sub while increasing its cost and size. A long drill string sub is undesirable between the motor and the bit because it adds additional flexibility to the assembly in this area which in turn makes the assembly more difficult to control. In addition, typical transmissions methods and devices operate at frequencies below 10 Hz which is too slow to support many of the recent active drill string components that require real time control information from the MWD system.
For these reasons, a method is required that can provide a communications link across drill string components such as a mud motor or rotary steerable using a means that can be implemented without weakening the structure of the drill string components while providing a high data transmission rate at low power.